The present invention relates to a combustion automatic control system for the flow or water temperature of water heaters such as boilers, with an expansion thermostat, acted by the water theater temperature, and placed in a control system housing, the thermostat being joined up by way of a lever linkage or the like in the form of a connection apparatus for controlling the position of the air inlet or fire door of the water heater, such door being oppositely acted upon by a spring force or by gravity. Such combustion automatic control systems have long been used, more specially as temperature controllers without any servo effect, for boilers and other water heaters fired by solid fuel, for example a boiler designed for solid fuel only, such as coke or coal, or one designed for two forms of fuel such as coke and oil or coke and gas. In the present specification the term "water heater" is used for describing a direct central heating boiler or an indirect boiler for producing hot water for other purposes than heating, in the home, in trade or in industry in a widely different number of designs. Such solid fuel fired water heaters have of late started becoming more and more important because of the oil shortfall.
An expansion thermostat placed within the control system housing and worked by the thermal expansion of a liquid, by vapor pressure or by gas absorption, is, more specially because of the use of an elastic metal bellows, as such not very strong in structue and likely to be damaged. The motion of the bellows, which is, generally speaking, small in size, is stepped up by a lever system or other linkage for producing a large enough control motion for driving the draft, air or fire door of the boiler, the force of the thermostat working in the opening-direction of the air or fire door against gravity, for example the weight of the door itself or against the force of a spring or other energy storing unit having the tendency of shutting the door. For this reason, a chain, joined with the thermostat is kept tight at all times in normal operation. For causing opening of the draft or fire door on a change in length of the thermostat, the system furthermore has parts producing forces or turning forces greater than the force of the door acting on the connection apparatus or linkage. Generally, such parts take the form of springs although in the past weights have been used as well. More specifically, a combustion automatic control system of this sort furthermore has a unit for adjustment of the desired temperature value, for example in the form of a turning head with a scale placed on the housing of the automatic control system and having within it the thermostat, the return spring and the linkage. The thermostat is generally placed within an immersed pipe-like housing within the water space of the boiler.
If in the case of such a combustion automatic control system there is a trouble condition with respect to the thermostat, for example because of its no longer being liquid-tight, it will be decreased in length and go into that position which a thermostat in full working order goes into when the water is cold. For this reason, the output signal of the thermostat seems to be that representative of a low temperature, with the outcome that the fire door is opened and this may be responsible for serious damage to the boiler if, for example, it is boiled dry.